The alkylation of aromatic hydrocarbons such as benzene is a well-developed art, and one that is practiced commercially using solid catalysts in large scale industrial units. Two common commercial applications are the production of ethyl benzene and cumene (isopropyl benzene). The production of ethyl benzene is the process of alkylating benzene with ethylene to produce ethyl benzene, which is the precursor used in the production of styrene. The production of cumene is the process of alkylating benzene with propylene to form isopropylbenzene, and which is used in the production of phenol. The production of ethyl benzene and cumene have undergone continual improvement, and an example of the process and typical flow scheme is shown in U.S. Pat. No. 4,051,191.
In the trans-alkylation of poly-ethylbenzene or poly-isopropylbenzene with an aromatic substrate, the issues of isomerization either do not exist or take place to a very minimal degree. Furthermore, the cyclization or cracking of the alkyl groups do not take place due to the lack of favorable mechanistic pathways. However, the situation is quite different in the trans-alkylation of poly-alkylated benzene, where the alkyl groups have 5 or more carbon atoms. Here the alkyl groups will undergo isomerization, followed by cyclization or cracking reactions. Cyclization of the alkyl groups results in multiple ring compounds, potentially accelerating the catalyst deactivation. The cracking of alkyl groups leads to light hydrocarbon products, potentially leading to lower yields and complicated separation situations. Due to the nature of consecutive reactions of cyclization and cracking processes, it would be beneficial to have the active sites confined to an outer layer to limit the diffusion path of the reactant and primary product, the linear alkylbenzene (LAB).
Therefore, improvements in the catalyst structure can make for more efficient processing while reducing the expense of the catalyst.